Characterizing B stars from Kepler/K2 Campaign 11

Abstract

Aims. In this study, we analyze 122 B-type star candidates observed during Campaign 11 of the Kepler/K2 mission to investigate their variability and pulsation characteristics. A subset of 45 B star candidates was observed during the Kepler/K2 mission’s Campaign 11 between September and December 2016. Our analyses aim to gain a deeper understanding of the physical characteristics of these massive stars. Our methods involve both spectroscopy and seismology. The spectroscopic analysis was performed through mediumresolution blue spectra, which also allowed us to perform a spectral classification of the objects. Our results will contribute to the ongoing effort to expand our knowledge of variable B stars and the processes that drive their variability. Methods. We used the iterative prewhitening and wavelet frequency searching algorithms to analyze the light curves to identify the different types of variability in the data. The frequencies were carefully chosen based on the signal-to-noise ratio and the magnitude of errors. We applied spectroscopic analysis techniques to enhance our understanding of the observed stars, including SME and MESA algorithms. A spectral classification was performed based on the observed spectra. The resulting astrophysical parameters were compared to Gaia mission data. Additionally, a seismology technique was applied to determine the average internal rotation frequency (vrot) and buoyancy travel time (P0) for selected stars in the sample. Results. We detected several types of variability among the B-type stars, including slowly pulsating B (SPB) stars, hybrid pulsators showing both β Cep and SPB pulsations, stars with stochastic low-frequency (SLF) variability, Maia variables, and SPB/Maia hybrids. Their positions in our Gaia and classical HR diagrams are compatible with the theoretical expectations. We also found stars exhibiting variability attributed to binarity and rotation. We determined the physical characteristics for 45 of our targets and conducted a seismic analysis for 14 objects. Two SPB/Maia stars show internal velocities comparable to those of fast SPB stars. The derived average rotation frequencies, vrot, for these 14 stars lie between the critical vcritRoche and the minimal frequency values of vlimrot implied by the υ sin i measured from the spectra. Conclusions. Our analysis classified 41 stars as SPB stars and attributed the primary variability of 53 objects to binarity, rotation, or both. We identified five stars as Maia/fast-rotating SPB variables. Two stars were classified as hybrid SPB/β Cep pulsators, and one as a β Cep binary. Thirteen stars exhibited prominent, low-frequency power excess, indicating SLF variability. Additionally, we found a positive correlation between the dominant fg frequency and the internal average rotation frequency.